Galacto-oligosaccharides (or Galactooligosaccharides) (GOS) are carbohydrates which are nondigestable in humans and animals comprising two or more galactose molecules, typically up to nine, linked by glycosidic bonds. GOS's may also include one or more glucose molecules. One of the beneficial effects of GOS's is their ability of acting as prebiotic compounds by selectively stimulating the proliferation of beneficial colonic microorganisms to give physiological benefits to the consumer. The established health effects have resulted in a growing interest in GOSs as food ingredients for various types of food.
The enzyme β-galactosidase (EC 3.2.1.23) usually hydrolyses lactose to the monosaccharides D-glucose and D-galactose. In the enzyme reaction of β-galactosidases, the enzyme hydrolyses lactose and transiently binds the galactose monosaccharide in a galactose-enzyme complex. Subsequently, water is used to hydrolyze the covalent galactose-enzyme intermediate resulting in the liberation of D-galactose and D-glucose. However, at high lactose concentrations some β-galactosidases are able to transfer galactose to the hydroxyl groups of D-galactose or D-glucose in a process called transgalactosylation whereby galacto-oligosaccharides are produced. At high lactose concentrations most β-galactosidases are able to transfer galactose to the hydroxyl groups of lactose or higher order oligosaccharides.
The genus Bifidobacterium is commonly used in the dairy industry. Ingestion of Bifidobacterium-containing products furthermore has a health-promoting effect. This effect is not only achieved by a lowered pH of the intestinal contents but also by the ability of Bifidobacterium to repopulate the intestinal flora in individuals who have had their intestinal flora disturbed by for example intake of antibiotics. Bifidobacterium furthermore has the potential of outcompeting potential harmful intestinal micro-organisms.
Galacto-oligosaccharides are known to enhance the growth of Bifidobacterium. This effect is likely achieved through the unique ability of Bifidobacterium to exploit galacto-oligosaccharides as a carbon source. Dietary supplement of galacto-oligosaccharides is furthermore thought to have a number of long-term disease protecting effects. For example, galacto-oligosaccharide intake has been shown to be highly protective against development of colorectal cancer in rats. There is a great interest in developing cheap and efficient methods for producing galacto-oligosaccharides for use in the industry for improving dietary supplements and dairy products.
An extracellular lactase from Bifidobacterium bifidum DSM20215 truncated with approximately 580 amino acids (BIF3-d3) has been described as a transgalactosylating enzyme in a solution containing lactose solubilised in water (Jørgensen et al. (2001), Appl. Microbiol. Biotechnol., 57: 647-652). WO 01/90317 also describes a truncation variant (OLGA347) as being a transgalactosylating enzyme and in WO 2012/010597 OLGA347 was shown to transfer a galactose moity to D-fucose, N-acetyl-galactosamine and xylose.
US2012/0040051 describes a process for preparing easily absorbable milk products with high galacto-oligosaccharide (GOS) content and low lactose content, and to a galactooligosaccharide-enhanced milk product prepared with the process using for example lactases from any origin, including, lactases from Aspergillus, Saccharomyces and Kluyveromyces. 
Galacto-oligosaccharide synthesis from a lactose solution or skim milk using the beta-galactosidase from Baccilus circulans is described in The Journal of Agricultural and Food Chemistry 2012, 60, 6391-6398.
WO2008/037839 discloses a process for producing products containing galacto-oligosaccharides by treating a milk-based raw material after addition of fructose and optionally lactose with a beta-galactosidase and terminating the enzymatic reaction of the reaction mixture.
EP0458358 discloses a skim milk powder containing galacto-oligosaccharide and a process for producing the same. The therein described process comprises adding beta-galactosidase to concentrated milk to give rise to an enzymatic reaction and heating the reaction mixture to 75-80° C. to terminate the enzymatic reaction followed by spraydrying of the reaction mixture.
US2006/0223140 discloses a transglycosylation method and a glycosidase having transglycosylation activity.
CN101396048 relates to a production method used for milk rich in galacto-oligosaccharide, comprising the steps as follows: heating milk, separating fat to get skim milk, pasteurizing, cooling, hydrolyzing by immobilizing β-galactosidase, UHT sterilizing, cooling and packaging.
In the present invention an efficient in situ conversion of low-concentration lactose to GOS is presented by treatment of milk-based medias using β-galactosidase such as a truncated lactase from Bifidobacterium bifidum DSM20215 consisting of 887 amino acids having SEQ ID no 1.
In addition, it has been found that the content of galacto-oligosaccharide(s) (GOS) produced in situ in a milk-based dairy application is not stable over time and is highly dependent on very low amounts of residual β-galactosidase activity within the milk-based product under normal storage conditions. This problem is also present in fermented dairy products such as yogurt with lowered pH, where the residual β-galactosidase activity is reduced even further. The β-galactosidase is highly stabilized in the milk matrix and a surprising high combination of treatment time and temperature compared to a buffered solution is required in the pasteurisation process to completely inactivate the β-galactosidase. Thus, the present invention describes a process that enables the use of a transgalactosylating β-galactosidase for in situ generation of stable GOS in different dairy products without negatively effecting important dairy product quality attributes, such as texture and flavour, due to residual β-galactosidase activity.